High data rate optical communication systems are subject to the effects of chromatic dispersion. Over long distances, compensation is necessary to support a predetermined bandwidth and bit error rate. Various techniques have been used to reduce the effects of chromatic dispersion. One common technique employs one or more dispersion compensating optical fibers to compensate for the chromatic dispersion resulting from transmission through a communication optical fiber. The length of dispersion compensating optical fibers depends on the length of communication optical fiber between the optical source and the receiver. It can be difficult to upgrade a communications network because of the variety of communication path lengths possible between optical sources and receivers. Moreover, dispersion compensation equipment represents a considerable expense and can result in an optical loss that may not be acceptable for high bit rate systems. However, as metro area networks move to higher data rates (e.g., 10 Gbps or greater), the need to effectively deal with chromatic dispersion has significantly increased.
Single sideband (SSB) transmission is a transmission method in which one of the two modulated sidebands generated on each side of a carrier frequency is suppressed and the other is transmitted. Thus the bandwidth required for data transmission is reduced by approximately one-half. Common to wireless communications, SSB transmission can also be applied to optical communications. SSB optical transmitters are beneficial to long distance optical communications systems because the impact of chromatic dispersion is substantially reduced in comparison to double sideband (DSB) optical transmitters so that high bit rate communications are possible without the need for dispersion compensating optical fiber.
SSB modulation of optical signals is realized through a variety of methods. In one example, a complex arrangement of multiple modulators is used to generate SSB optical signals, see Shimotsu et al., “Development of Optical SSB Modulator,” Sumitomo Osaka Cement Co., Ltd., Technical Report 2001, pp. 4-7, http://www.socnb.com/report/ptech_e/2001p06_e.pdf. In another example, multiple drive signals are used to control a single modulator in an SSB transmitter, see U.S. Pat. No. 5,239,401 to Olshansky. However, such systems are typically expensive and require complex feedback systems.
SSB optical signals can also be generated by applying a current modulated data signal to the gain module of a semiconductor laser and intensity modulating the resulting laser output beam as disclosed in U.S. Pat. No. 5,999,300 to Davies et al. However, the current modulated data signal according to this technique also imparts an unwanted intensity modulation to the laser output beam emitted by the semiconductor laser, resulting in incomplete cancellation of one sideband. Consequently, the performance of optical communications systems employing transmitters using this technique is limited.